The present invention relates to a method for removing ink-accepting areas of a printing master so that its substrate can be recycled and used again for applying a coating thereto.
Printing presses use a so-called printing master such as a printing plate which is mounted on a cylinder of the printing press. The master carries an image consisting of ink-accepting areas on its surface and a print is obtained by applying ink to said image and then transferring the ink from the master onto a receiver material, which is typically paper. In conventional lithographic printing, ink as well as an aqueous fountain solution (also called dampening liquid) are supplied to the lithographic image which consists of oleophilic (or hydrophobic, i.e. ink-accepting, water-repelling) areas as well as hydrophilic (or oleophobic, i.e. water-accepting, ink-repelling) areas. In so-called driographic printing, the lithographic image consists of ink-accepting and ink-abhesive (ink-repelling) areas and during driographic printing, only ink is supplied to the master.
Printing masters are generally obtained by the so-called computer-to-film method wherein various pre-press steps such as typeface selection, scanning, color separation, screening, trapping, layout and imposition are accomplished digitally and each color selection is transferred to graphic arts film using an image-setter. After processing, the film can be used as a mask for the exposure of an imaging material called plate precursor and after plate processing, a printing plate is obtained which can be used as a master.
In recent years the so-called computer-to-plate (CTP) method has gained a lot of interest. This method, also called direct-to-plate method, bypasses the creation of film because the digital document is transferred directly to a plate precursor by means of a so-called plate-setter. A special type of CTP processes involves the exposure of a plate precursor while being mounted on a plate cylinder of a printing press by means of an image-setter that is integrated in the press. This method may be called xe2x80x98computer-to-pressxe2x80x99 and printing presses with an integrated plate-setter are sometimes called digital presses. A review of digital presses is given in the Proceedings of the Imaging Science and Technology""s 1997 International Conference on Digital Printing Technologies (Non-Impact Printing 13). Computer-to-press methods have been described in e.g. EP-A 640 478, EP-A 770 495, EP-A 770 496, WO 94/1280, EP-A 580 394 and EP-A 774 364.
Two types of such on-press imaging methods are known. According to a first type, a printing plate precursor is mounted on a printing press, image-wise exposed, optionally developed, and then used as a printing master and finally removed from the press and disposed of, thus requiring a new plate material for each image. An example of this technology is the Quickmaster DI 46-4 (trade mark of Heidelberger Druckmaschinen, Germany). A drawback of this method is the need to use a new plate for each press run, thus increasing the cost of the printing process.
In a second type of on-press imaging systems, the same lithographic substrate is used in a plurality of press runs (hereinafter called printing cycles). In each printing cycle, one or more heat-sensitive or photosensitive layer(s) are coated on the lithographic substrate to make a printing plate precursor and after image-wise exposure and optional development a printing master is obtained. After the press-run, the ink-accepting areas of the printing master are removed from the lithographic substrate in a cleaning step so that the substrate is recycled and can be used in a next cycle of coating, exposing and printing without the need to mount a new plate on the cylinder. Examples of such on-press coating and on-press imaging systems are described in e.g. U.S. Pat. No. 4,718,340; U.S. Pat. No. 5,188,033; U.S. Pat. No. 5,713,287; EP-A 786 337 and EP-A 802 457. The substrate can be reused during a number of printing cycles, which is dependent on the delicate balance between the effectiveness of the cleaning step and the preservation of the lithographic quality of the substrate: an aggressive cleaning step effectively removes all traces of the lithographic coating but may attack the lithographic surface of the substrate; a mild cleaning step, on the other hand, reduces the risk of affecting the lithographic quality of the substrate but incomplete removal of the lithographic coating typically results in the appearance of ghost images (the image of a previous print job is visible in the next). In practice, the same substrate cannot be used indefinitely and needs to be replaced after a number of print cycles.
U.S. Pat. No. 4,718,340 discloses a method wherein the lithographic coating is removed from the substrate by laser ablation. The very high temperature that is generated in the ablated lithographic coating, however, damages the lithographic surface of typical substrates such as grained and anodized aluminum.
It is an object of the present invention to provide a method for effectively cleaning the substrate of a printing master without affecting the quality of the substrate, so that it can be reused in a next printing cycle without the appearance of ghost images. This object is realized by the method for removing ink-accepting areas from a printing master by laser ablation, wherein the printing master comprises a substrate which comprises a support and a base layer, wherein the base layer contains a crosslinked hydrophilic binder and a metal oxide. Although applicable to other printing techniques such as letterpress printing, the method is especially suited for cleaning a lithographic printing master.
In a preferred embodiment, the present invention discloses a method of lithographic printing with a reusable substrate by
(a) providing a substrate comprising a support and a base layer which contains a crosslinked hydrophilic binder and a metal oxide;
(b) applying one or more layer(s) on the base layer, thereby obtaining an imaging material;
(c) making a printing master having ink-accepting areas by image-wise exposure of the imaging material to heat or light and optionally processing the imaging material;
(d) printing;
(e) removing the ink-accepting areas from the printing master by laser ablation; and
(f) repeating steps (b) through (d).
This cleaning method is used for recycling the substrate in an on-press coating and on-press exposure method. The method allows effective removal of ink-accepting areas from the substrate of the printing master (no ghost images in the subsequent printing cycle) without affecting the lithographic quality of the substrate, thereby allowing to use the same substrate in a large number of printing cycles of coating, exposing, printing and cleaning, said number being preferably larger than 5, more preferably larger than 10 and most preferably larger than 30.